Cluster model approach for electronic structure of Si and Ge(111) and GaAs(110) surfaces

For the purpose of exploring how realistic a cluster model can be for semiconductor surfaces, extended Huckel theory calculations are performed on clusters modeling Si and Ge(111) and GaAs(110) surfaces as prototypes. Boundary conditions of the clusters are devised to be reduced. The ideal, relaxed, and reconstructed Si and Ge(111) surfaces are dealt with. Hydrogen chemisorbed (111) clusters of Si and Ge are also investigated as prototypes of chemisorption systems. Some comparison of the results with finite slab calculations and experiments is presented. The cluster-size dependence of the calculated energy levels, local densities of states, and charge distributions is examined for Si and Ge(111) clusters. It is found that a 45-atom cluster which has seven layers along the [111] direction is large enough to identify basic surface states and study the hydrogen chemisorption on Si and Ge(111) surfaces. Also, it is presented that surface states on the clean Si and Ge(111) clusters exist independent of relaxation. Further, the calculation for the relaxed GaAs(110) cluster gives the empty and filled dangling-orbital surface states comparable to experimental data and results of finite slab calculations. The cluster approach is concluded to be a highly useful and economical one for semiconductor surface problems.     

采用(meta)-GGA泛函对Ag(111)、Rh(111)和Ir(111)上乙烯吸附的理论研究

本文针对在多种催化反应的重要中间体乙烯,使用(meta)-GGA等级的包含PBE,BEEF-vdW,SCAN以及SCAN+rVV10在内的多种交换关联泛函,系统研究了在过渡金属表面(Ag,Rh和Ir)上乙烯吸附势能面对泛函的依赖关系. 研究发现,对于乙烯在贵金属Ag(111)上的吸附,除了PBE外,BEEF-vdW,SCAN以及SCAN+rVV10均能预测出物理吸附态的存在. 对于乙烯在Rh(111)面的吸附,SCAN和SCAN+rVV10预测在化学吸附位之前存在有物理吸附前驱态,而基于PBE和BEEF-vdW的势能面并没有发现前驱态的存在. 而对于乙烯在Ir(111)上的吸附,BEEF-vdW也能微弱地预测出化学吸附前驱态的存在. 研究结果表明,无论在哪一种金属表面上,四种泛函中SCAN+rVV10给出的吸附能最强,其次是SCAN,最后是PBE或者BEEF-vdW.     

Pyridine chemisorption on silicon and germanium (111) surfaces

The chemisorption of pyridine molecules on cleaved Si(111) and Ge(111) surfaces was investigated by ultraviolet photoemission spectroscopy with synchrotron radiation. Evidence was found that the chemisorption process strongly affects all the three highest occupied molecular levels, i.e. the n-level and the two π-levels la₂ and 2b₂. This result was used to rule out a chemisorption geometry with the aromatic ring parallel to the surface. In the most likely chemisorption geometry the ring is tilted with respect to the substrate and the n-orbital plays a leading role in the formation of chemisorption bonds.     

Calculation of geometries and chemisorption energies of adatoms on simple metals

We report results of calculations with a formalism that in principle applies quite generally for chemisorption on a real metallic substrate. Including the substrate structure within perturbation theory on a self-consistent jellium-plus-adatom calculation, we have computed the dependence of the binding energy of an adatom on the surface geometry. Specifically, in the case of hydrogen on Al, our model calculation predicts that the stable positions are bridge configurations on the (100) and (110) surfaces and atop positions on the (111) surface, and that they have almost the same heats of chemisorption (1.8–2.0 eV). For geometrical reasons the bridge configuration seems to be a reasonable result while the atop result for the (111) surface is more uncertain. Thus, chemisorption of H on Al should require predissociation of the H₂ gas. In addition, the predicted values for hydrogen desorption imply that measurements on H on Al surfaces should be performed at low temperatures to avoid desorption. Results for H on a jellium of Na density indicates that hydrogen should be absorbed in rather than adsorbed on Na metal.     

Co chemisorption on PtCu surfaces III. CO on PtCu(111)

Selected thermal desorption and valence band photoemission data on the chemisorption of CO on PtCu(111) surfaces are presented. The main objective is to make a comparison with CO chemisorption on an annealed (1 × 3) reconstructed Pt_0.98Cu_0.02(110) surface. The (111) alloy surfaces are unreconstructed (1 × 1) surfaces, with average near-surface Cu concentrations ranging from ? 7.5% to ? 20% as indicated by the Cu 920 eV Auger signal. It is observed that the effect of alloying Pt(111) with Cu is to progressively lower the desorption peak temperature and hence the free energy of CO desorption from Pt sites. A second observation is that the energy distribution of the Cu 3d-derived states is little affected by CO adsorption on Cu sites at 155 K. Both these results offer a contrast to the results for CO/Pt_0.98Cu_0.02(110) reported earlier.     

The chemisorption of oxygen,water and selected hydrocarbons on the (111) and stepped gold surfaces

The dissociative chemisorption of oxygen and water is reported on both (111) and [6(111) × (100)] crystal faces of gold. The oxide formation becomes rapid above 500°C at pressures of about 10^?6 torr. The resulting gold oxide is bound strongly. It is similar in structure to the corresponding sulphide and is stable on both surfaces to 800°C in vacum. Ethylene, cyclohexene, n-heptane, benzene did not chemisorb on gold under low pressure conditions on either the (111) or on the stepped gold surface while naphthalene exhibited dissociative chemisorption on both types of surfaces. Hydrocarbon fragments are bound strongly to the gold surface but the activation energy for dissociative adsorption of light hydrocarbon molecules appears to be high.     

ESR and TPD Investigations of the Adsorption of Di-tert-butyl Nitroxide on Au(111) and NiO(111). Evidence for Long-Range Interactions

Electron spin resonance and temperature-programmed desorption spectra of thin layers of DTBN (di-tert-butyl nitroxide) adsorbed on Au(111) and NiO(111)/Au(111) surfaces have been measured. The temperature-programmed desorption data show a weak chemisorption of the DTBN molecules in the monolayer on both surfaces. On Au(111) as well as on NiO(111)/Au(111), the ESR signal from monolayer coverages is totally suppressed. This suppression continues into the multilayer regime on both substrates. Disturbances of the substrate/adsorbate interface have a strong influence on the range of the signal suppression. Possible reasons for this behavior are discussed.     

Chemisorptive bonding and reaction of nitric oxide on Ir(100) and Ir(111) surfaces

Chemisorption of nitric oxide on single crystal Ir(111) and Ir(100)?(5 × 1) has been studied by UV-photoelectron spectroscopy, thermal desorption and low energy electron diffraction. At 300 K, partially dissociative adsorption is observed on both surfaces, confirming the borderline location of Ir in the Periodic Table with respect to molecular versus dissociative adsorption. Three different molecular chemisorption phases are distinguished in the UPS spectra through distinctly different 1π-level energies. A skewed orientation associated with a possible rehybridization and bending of the nitrosyl-metal bound for chemisorption on the Ir(111) surface is inferred both from a splitting of the 1π level and from observation of relative intensity variations in photoemission using a polarized photon source.     

Surface Raman spectroscopy without enhancement: Pyridine on Ag(111)

We have observed normal Raman scattering from a monolayer of pyridine adsorbed on a Ag(111) surface at 110 K. Unlike many previous studies of this system, we find no appreciable enhancement of the scattering cross section. Our results suggest that the short range enhancements observed on other well-characterized silver surfaces may be due to chemisorption on sites that are not available on the (111) surface.     

Study of chemisorption on silver surfaces using ultraviolet photoelectron spectroscopy (UPS)

Ultraviolet photoelectron spectroscopy (UPS) was used to study the chemisorption of halogens on stepped [3(111) × (100)] and low-index (111) silver surfaces. The initial rate of halogen adsorption using CHCl₃ exposure on the silver stepped surface is approximately twice that on the low-index surface. This indicates that steps play an important role in chemisorption even on metals with a low density of states at the Fermi level. The adsorbate-induced levels on silver were correlated with halogen p valence orbitals using model extended Hückel calculations. Changes in the silver d band are interpreted as due to p?d orbital interactions.     

Comparative study of C-H stretch and bend vibrations in methane activation on Ni(100) and Ni(111)

State-resolved measurements on clean Ni(100) and Ni(111) surfaces quantify the reactivity of CH4 excited to v = 3 of the nu4 bend vibration. A comparison with prior data reveals that 3nu4 is significantly less effective than the nu3 C-H stretch at promoting dissociative chemisorption, even though 3nu4 contains 30% more energy. These results contradict statistical theories of gas-surface reactivity, provide clear evidence for vibrational mode specificity in a gas-surface reaction, and point to a central role for C-H stretching motion along the reaction path to dissociative chemisorption.     

Leakage of O2 precursor molecules from inert hydrogen islands on a Pt(111) surface

We have observed, using infrared spectroscopy, that the precursor-mediated O2 chemisorption on the clean and the partially hydrogen-covered Pt(111) surfaces exhibits opposite temperature dependencies above the temperature for stable O2 physisorption. While the chemisorption probability on the clean surface increases with increasing temperature due to thermal activation of the precursor, it decreases on the partially hydrogen-covered surface which we suggest is due to a general loss of the mobile precursor molecules by thermal desorption from chemically inert hydrogen islands.     

Nature of versatile chemisorption on TiC(111) and TiN(111) surfaces

Density-functional calculations on the polar TiX(111) (X = C, N) surfaces show (i) for clean surfaces, strong Ti3d-derived surface resonances (SR’s) at the Fermi level and X2p-derived SR’s deep in the upper valence band and (ii) for adatoms in periods 1-3, pyramidic trends in atomic adsorption energies, peaking at oxygen (9 eV). A concerted-coupling model, where adatom states couple to both kinds of SR’s in a concerted way, describes the adsorption. The chemisorption versatility and the general nature of the model indicate ramifications and predictive abilities in, e.g., growth and catalysis.     

Ultraviolet photoemission studies of acetylene and ethylene adsorptions on the Pt(111) surface: Correlations with LEED studies

The chemisorption of acetylene and ethylene on platinum (111) surfaces for T ≥ 300 K has been studied with ultraviolet photoelectron spectroscopy (UPS) at 21.2 eV. An activated metastable-stable acetylene transition observed recently in low-energy electron diffraction (LEED) intensity-energy profiles has been seen with the UPS spectra. The upperlying electronic levels of the metastable acetylene state are related to a shifted gas-phase acetylene spectrum. The stable acetylene state appears to involve a stronger molecule-surface interaction and probable rehybridization, consistent with the LEED analysis showing the molecule to be situated in a triangular position at covalent Pt-C distances. Ethylene is founf to dehydrogenate at room temperature to the stable acetylene species on Pt(111) surfaces.     

The process of oxygen chemisorption on the Si(111) surface

The chemisorption of oxygen on the Si(111) surface has been studied by the ASED-MO method. Three steps of the initial oxidation process have been proposed. The first step is molecular oxygen chemisorption. The second step is that of dissociated oxygen chemisorption in which the atomic short bridge site (between the first layer and second layer silicon atoms) can be occupied only after the saturation of the dangling bonds of the surface silicon with oxygen. The third step is the diffusion of atomic oxygen from the short bridge positions into the bulk of silicon to form an SiO₂ film. For molecular chemisorption, both the peroxy vertical and peroxy bridge models are possible although the peroxy vertical model is the more stable. The dissociated atomic oxygen can chemisorb for both the on-top and the short bridge models. Our results can explain, and are consistent with, most experimental results.     

Methanol decomposition on platinum (111)

The interaction of methanol with clean and oxygen-covered Pt(111) surfaces has been examined with high resolution electron loss spectroscopy (EELS) and thermal desorption spectroscopy (TDS). On the clean Pt(111) surface, methanol dehydrogenated above 140 K to form adsorbed carbon monoxide and hydrogen. On a Pt(111)-p(2 × 2)O surface, methanol formed a methoxy species (CH₃O) and adsorbed water. The methoxy species was unstable above 170 K and decomposed to form adsorbed CO and hydrogen. Above room temperature, hydrogen and carbon monoxide desorbed near 360 and 470 K, respectively. The instability of methanol and methoxy groups on the Pt surface is in agreement with the dehydrogenation reaction observed on W, Ru, Pd and Ni surfaces at low pressures. This is in contrast with the higher stability of methoxy groups on silver and copper surfaces, where decomposition to formaldehyde and hydrogen occurs. The hypothesis is proposed that metals with low heats of adsorption of CO and H₂ (Ag, Cu) may selectively form formaldehyde via the methoxy intermediate, whereas other metals with high CO and H₂ chemisorption heats rapidly dehydrogenate methoxy species below room temperature.     

Crystal field surface orbital-bond energy bond order (CFSO-BEBO) calculations of adsorption: II. Carbon monoxide,oxygen and carbon dioxide on platinum (111) and oxygen on nickel (111)

The Crystal Field Surface Orbital-Bond Energy Bond Order (CFSO-BEBO) model of chemisorption is applied to the interaction of carbon monoxide, oxygen and carbon dioxide with a (111) platinum surface; and the interaction of oxygen with a (111) nickel surface. No activation energy for molecular adsorption of carbon monoxide on platinum is predicted; however a large activation energy for dissociative chemisorption is calculated. The molecular state has a binding energy of approximately 28 kcal/mole, and vibrational stretching frequencies of 1935 and 1975 cm^?1 are calculated by combining the CFSO-BEBO model with Badger's Rule. The adsorption of oxygen on (111) platinum and (111) nickel are predicted to be different in the following respects: (1) There is an activation energy of 2.1–4.5 kcal/mole on platinum, whereas adsorption on nickel is unactivated; and (2) The dissociative heat of chemisorption on platinum is 58–68 kcal/mole, whereas on nickel it is substantially larger, 112–116 kcal/mole. The adsorption of carbon dioxide on (111) platinum is predicted to be not only highly activated but also endothermic. All of the calculated results are essentially in quantitative agreement with available experimental data.     

Vibrations spectra of oxygen chemisorbed on nickel (110) surfaces

Using a simple tight-binding scheme to describe the nickel d states and the oxygen p states, we calculate the positions and the vibration frequencies of chemisorbed oxygen atoms on nickel (110) surfaces. The comparison between our results and the high resolution electron loss measurements suggests a longbridge chemisorption site at low and high oxygen coverages on the nickel surface.     

LCAO studies of hydrogen chemisorption on nickel: I. Tight-binding calculations for adsorption on periodic surfaces

The model we have used to study hydrogen chemisorption on nickel surfaces is a tightbinding Extended Hückel method applied to finite (periodic) crystals up to about 250 atoms, the non-orthogonal basis set comprising five 3d orbitals, one 4s orbital and three 4p orbitals per atom. After calculating the band structure of fcc nickel, we have examined, by this model, the effect of the (100), (110) and (111) surfaces on the local density of states and the charge distribution. The results agree closely with moment calculations of the density of states in semi-infinite crystals and with experimental (XPS, UPS and INS) spectra. Extensive studies have been made of the influence of adsorption on the (partial) densities of states in order to illuminate the nature of the chemisorption bond. Particularly, we have concluded that both the 3d electrons and the conduction electrons take part in this bond. Equilibrium positions for adsorption on various sites have been determined and the adsorption energy has been computed and compared with experimental data. We find that the stability of adsorption decreases in the order (110) > (100) > (111) and Atop > Bridge > Centred.     

Leed and thermal desorption studies of small molecules (H2, O2, CO,CO2, NO,C2H4, C2H2 AND C) chemisorbed on the rhodium (111) and (100) surfaces

The chemisorption of H₂, O₂, CO, CO₂, NO, C₂H₄, C₂H₂ and C has been studied on the clean Rh(111) and (100) surfaces. LEED, AES and thermal desorption were used to determine the surface structures, disordering and desorption temperatures, displacement and decomposition characteristics for each species. All of the molecules studied readily chemisorbed on both surfaces. A large variety of ordered structures was observed, especially on the (111) surface. The disordering temperatures of most ordered surface structures on the (111) surface were below 100°C. It was necessary to adsorb the gases at 25° C or below in order to obtain well-ordered surface structures. Chemisorbed oxygen was readily removed from the surface by H₂ or CO gas at crystal temperatures above 50°C. CO₂ appears to dissociate to CO upon adsorption on both rhodium surfaces as indicated by the identical ordering and desorption characteristics of these two molecules. C₂H₄ and C₂H₂ also had very similar ordering and desorption characteristics and it is likely that the adsorbed species formed by both molecules is the same. Decomposition of ethylene produced a sequence of ordered carbon surface structures on the (111) face as a result of a bulk-surface carbon equilibrium. The chemisorption properties of rhodium appear to be generally similar to those of iridium, nickel and palladium.